1. Field of the Invention
The present invention relates to a control system of an alternating current generator for charging battery for controlling output voltage of that generator so that the voltage becomes to a voltage of a predetermine valve.
2. Description of the Prior Art
FIG. 4 shows a circuit of a control system of a conventional alternating current generator, and this shows a circuit in the case where it is employed in an alternating current generator being used for vehicle.
1 is an alternating current generator comprising three generator coils 101 and a magnetic field coil 102. 2 is a full wave rectifier for rectifying output of the generator coils 101 and consisting of six diodes and having an output terminal 201 on the positive electrode side and a grounded output terminal 202 on the negative electrode side.
3 is a voltage control circuit being connected in series to the negative electrode side of the magnetic field coil 102 and this control circuit consists of a constant voltage power supply A, which consists of a resistance 301 for limiting electric current and a zener diode 302 for fixing voltage to a fixed one, potential dividing resisters 303 and 304 for detection of the output voltage of the generator 1, a comparator 305 for comparing the detection voltages appearing through the potential dividing resistances 303 and 304 with the predetermined voltage Ref1, a power transistor 308 serving as a first power transistor, resistance 306 for supplying base current and a suppression diode 307 for absorbing surge voltage of the magnetic field coil.
4 is a battery whose negative side is grounded and whose positive side is connected to one end of a key switch 5, to one end of the resistance 303, to cathode of the diode 307, to positive electrode side of the magnetic field coil 102 and to the output terminal 201 on the positive electrode side of the full wave rectifier 2.
The predetermined voltage Ref1 serving as a first predetermined value is supplied by dividing the output voltage of the constant voltage power supply A by means of unshown resistors, and in this case, and this divided voltage is set to about 14.5V as a threshold voltage for switching output voltage of the comparator 305 to "High" or "low".
Detailed description on the constitution of the voltage control circuit 3 will be given subsequently. One end of the resistance 301 is connected to another end of the key switch 5 and to one end of the resistance 306, and another end of the resistance 301 is connected to the cathode of the zener diode 302 and becomes to a feeding terminal of the constant voltage power supply A. Anode of the zener diode 302 is grounded. One end of the resistance 303 is connected to the positive electrode side of the battery 4 and its another end is connected to one end of the resistance 304 whose another end is grounded. A portion between the resistances 303 and 304 is connected to the negative input electrode of the comparator 305. The predetermined voltage Ref1 is supplied to the positive electrode side of the comparator 305. The output terminal of the comparator 305 is connected to the another end of the resistance 306 and to the base of the power transistor 308. Collector of the power transistor 308 is connected to the anode of the suppression diode 307 and to the side of the negative electrode of the magnetic field coil 102 and its emitter is grounded.
Subsequently, operation of the control circuit will be described. When a driver turns on the key switch 5, a predetermined voltage is generated at the constant voltage power supply A by the electric current flowing through the resistance 301 and the zener diode 302 resulting in generation of a predetermined voltage at the constant voltage power supply A, and thus supplied predetermined voltage Ref1 is input to the comparator 305. At this time, because the generator 1 has not yet generated electricity, the output voltage of the battery 4 detected through the resistances 303 and 304 is lower than the predetermined constant voltage Ref1. Accordingly, output of the comparator 305 becomes "High" and in turn the transistor 308 is turned on and thus the magnetic field current flows trough the magnetic field coil 102 caused by turning on the transistor 308.
By turning on the starting switch (unshown) thereafter, the engine is started and the alternating current generator 1 starts the alternating current generation. Output of alternating current generator 1 is rectified by the full wave rectifier 2 and the battery is charged.
At this time, when the output voltage of the alternating current generator 1 is lower than the predetermined voltage Ref1, this lower voltage is detected through resisters 303 and 304 for potential division, and the output of comparator 305 becomes "High" and the power transistor 308 becomes conductive and thus the current flowing through the magnetic field coil 102 is increased resulting in rising up of the output voltage of the generator 1. When this output voltage exceeds the predetermined voltage Ref1, operation contrary to the aforementioned is carried on. In other words, output of the comparator 305 becomes "Low" and the power transistor 308 is shut off and this causes reduction of the magnetic field current of the magnetic field coil 102 and in turn the output voltage of the alternating current generator 1 decreases.
After that, by repetition of the above mentioned operations, the control circuit controls the output voltage of the alternating current generator 1 so that it maintains a predetermined value. In this case, the magnetic field current to the magnetic field coil 102 is controlled by turning on and turning off it so as to enabling the battery 4 to maintain its voltage to the predetermined voltage Ref1 of 14.5V.
However, in the aforementioned conventional system, suppose should a trouble is developed such that the power transistor 308 is shorted or the negative side of the magnetic field coil 102 is grounded by an extraneous metal and the like, the voltage control circuit 3 is obliged to be bypassed and this makes the magnetic field current of the magnetic field coil 102 to be uncontrollable and as a consequence the maximum magnetic field current is to flow through the magnetic field coil 102; in turn the output voltage of the alternating current generator 1 rises unusually to an over voltage and this over voltage inevitably destroys electric loads (unshown) on board the vehicle. Further, due to this over voltage, there was a possibility of destroying devices being necessary for vehicle running and thus the vehicle will be able to run no longer.
A control system with a function of displaying overvoltage of a battery 4 disclosed in the Japanese Patent Publication No. 2579812 is known; but this system has no ability of recovery from the overvoltage at the time when the battery 4 becomes to be over voltage and further has no ability of continuing the battery charging and electricity supply to the electric loads because of stoppage of the generation of electricity.
The present invention has been made in order to solve the forgoing problems, and the object of the invention is to control, even when a malfunction is developed on the negative electrode side of the magnetic field coil, the output voltage of the alternating current generator is controlled so that the operation of charging battery and that of supplying electricity to the electric loads can be continued.